Hi,
AFAIK, Rato-co doesn't yet have a replacement for the TR4 "ladder" chassis, just for TR4A and later models.
If they do, that would be the only source for a new part on this side of the Atlantic. I think there are one or two British vendors who might make up semi-custom built one (be sitting down when you ask the price, and that's before shipping and import duties!). Probably Revington TR could handle it, possibly some others as well.
Better bet would probably be to track down a good, used one. It's not that uncommon, the TR4 chassis was made of heavier metal and tends to hold up better than later TR frames. Also, after a friend cut one up for scrap (bent beyond repair), he told me he found the inside of it was painted by the factory, prior to assembly and tack welding. That might be another reason the earlier frames tend to outlive the later ones.
There were some minor differences between TR4 chassis themselves. You will find a plate on the main chassis member under the front of the engine. My '62 TR4 had a "C" frame. When I rebuilt it, I got hold of a used, "B6" (or maybe "B8", I can't recall exactly) from a slightly earlier car. To give you some idea how minor the differences were, I had to add gussets to the rack & pinion mounts and an exhaust bracket, which apparantly weren't used on the earlier cars. A little bending and welding was all that was needed. I can only assume there were "A" "B" and "C", possibly others and some numerical suffixes within those. Just watch out if your car is a super early one with the 0 degree caster front end, in which case the frame parts might be significantly different and the front portion around the spring towers would likely be the most difficult to swap parts around, where the dimensions are pretty critical. If your car were a very early one, a TR3 frame might even be modified to fit. (Triumph did so at the factory themselves.) Front and rear body mountings are different, but are available from some vendors. Outriggers need to be lengthened about an inch each side. Rack & pinion mountings need to be installed.
Later TR4 ladder chassis used a different R&P and mounts for it, too. Same parts as were used on TR4A - TR6.
So, in light of some of the changes, the easiest replacement would be one from a car with the matching frame designation, which would be one with a similar or relatively close commission number. OTOH, with a few exceptions, differences are pretty easily rectified.
You say yours is rotted, but are you sure it's not repairable? Without more detail about just what's rusted away, I can't really say for sure.
Again, Revington TR is a major supplier of many pieces to repair the frames. Most vendors offer some of the more commonly needed parts. Much of it is also pretty simple stuff for a local welding shop to fabricate from scratch. Might be worth checking out, before you spend very much on a used replacement.
Yes, as noted, the body is designed to be lifted off pretty easily, and 4 guys can do it. I did it single-handedly... okay, I used a couple 1000 lb chain hoists. The tub itself probably weight 400 lbs or there-abouts. The more that is stripped off, the easier it is to lift, of course! If the doors are removed, you must brace it, so the body doesn't try to fold in half under it's own weight (usually just gets tweaked and twisted, unless severely rotted in the rockers and elsewhere).
Yes, I agree... get the factoy service manual for a detailed description of the process of removing the body and guidelines for lifting it. There are a couple photos of the body lifted off my TR4 at the link in my signature lines.
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